Do not delete objdump when WITH_LLVM_BINUTILS is true

WITH_LLVM_BINUTILS links /usr/bin/objdump to llvm-objdump, and similarly
for the man page. Do not delete them in make delete-old.

PR: 266603
Sponsored by: The FreeBSD Foundation

(cherry picked from commit d1a351be19d4bd03918d44ea33fa7c09655f810a)